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Sharing secrets Superb fairy wren chicks are learning the importance of listening to their mother before they've even hatched, a new Australian study has found.

According to the study, published today in journal Cell Biology, superb fairy wren mothers teach a "password" to their young while they are still in the egg as a means of identifying them when they want to be fed and stopping home invasion.

"Parents and others attending the nestlings will only feed them if their begging calls contain the learned password," says co-author Professor Sonia Kleindorfer of Flinders University in Adelaide.

Otherwise, the parents simply abandon the nest and start again.

Kleindorfer says it is the first time prenatal learning has been uncovered in birds.

She says the discovery was made by accident while the research team was examining the effects of predation on the fairy wren.

While analysing sound and video footage from a series of fairy wren nests in the South Australian wild, the researchers noticed the fairy wren mothers appeared to be singing to their unhatched eggs.

Kleindorfer says the female would call repeatedly - often as much as 15 times an hour.

"We then found when the nestlings hatched the chicks had a one-note begging call," says Kleindorfer. An analysis of the mother's "incubation call" showed it contained this begging note used by the chicks.

The researchers also determined that each nest had a different "password".

To determine if the trait was genetic or learned, the team then swapped eggs between 22 nests.

In swapped nests the chicks cry was more similar to its foster mother than biological mother, Kleindorfer says, indicating the trait is learned.

Outsmarting interlopers

She says the feeding password plays an important role in helping fairy wren mothers identify the chicks of parasitic birds such as cuckoos that abandon their parenting role by laying their eggs in the fairy wren's nest.

The cuckoo chicks hatch earlier and monopolise their new parents by ejecting rival fairy wren siblings eggs from the nest.

However the "password" helps the fairy wren mother identify her chicks and reject the home invader as she begins "teaching" her chicks the feeding call about 10 days after the eggs are laid.

Cuckoo chicks begin hatching about one day later whereas the fairy wren chicks have another four to five days in the egg.

Kleindorfer says this means the parasitic cuckoo chick does not have time to learn the feeding call.

The researchers also believe the prenatal learning has other benefits for the fairy wrens and plan to delve deeper into the finding.

Maintaining maternal lines

Kleindorfer says because the fairy wren is notoriously promiscuous, the male of the nest may not be the chicks' father.

"If the female can transmit these small bits of culture or messages to the young it is creating a maternal lineage," she says.

The researchers are also studying the cost of the calls in terms of predation by examining the rate of calls by the mother in the teaching phase.

Kleindorfer suspects in areas of high predation of the superb fairy wrens by other birds such as currawong and magpies the mothers would reduce the number of calls per hour to the eggs to reduce the chances of attack.